1. Field of the Invention
The invention relates to a low pressure discharge lamp, and especially to a low pressure discharge lamp which is used for backlighting of a liquid crystal display device, for a light source for a scanning device, for a light source for purposes of general illumination, for a UV light source and the like.
2. Description of Related Art
Recently liquid crystal display devices have been used in various areas. For backlighting of liquid crystal display devices, fluorescent lamps which are a type of low pressure discharge lamp have been used to advantage.
Furthermore, low pressure discharge lamps have also been used recently for the light source of a scanning device, a UV light source, and the like.
Low pressure discharge lamps are divided into so-called low pressure discharge lamps of the hot cathode type, in which mainly the phenomenon of thermal electron emission is used, and into so-called low pressure discharge lamps of the cold cathode type, in which mainly the phenomenon of secondary electron emission is used. The low pressure discharge lamps of the cold cathode type have no luminous filaments for heating, and the operating temperature (temperature during operation) of its electrodes is less than or equal to roughly 700 K. On the other hand, the operating temperature of the electrodes of low pressure discharge lamps of the hot cathode type is, for example, roughly 1000 K. The operating temperature of the electrodes of short arc discharge lamps which are not considered to be low pressure discharge lamps reaches roughly 2500 K.
However, in the low pressure discharge lamps of the cold cathode type, the cathode drop voltage during operation (the potential difference in the middle area between the electrode surface and the positive column (plasma)) is large, for example, greater than or equal to 120 V, if their electrodes are not provided with electron emission material. As is apparent therefrom, the disadvantage arises that the portion of the total power consumed in the entire lamp which is constituted by the power consumed by the electrodes themselves, i.e. the power which does not contribute to emission, is large, and as a result, the radiant efficiency with respect to power consumption is low.
Therefore, there are cases in which, even in low pressure discharge lamps of the cold cathode type, the electrodes are provided with electron emission material. In this case, as in the electrodes of low pressure discharge lamps of the hot cathode type or the like, electron emission material which is comprised primarily of barium (Ba) and the like is applied to the surface of the metal electrode substrate by coating or burning in. In the case in which the electrode substrate is provided with electron emission material, due to the presence of this electron emission material, the cathode drop voltage is, for example, roughly 60 V. This reduces the power consumption on the electrodes and the radiant efficiency with respect to power consumption is therefore increased.
However, the electron emission material, such as barium or the like, often sprays due to sputtering by ions and electrons which form in the glass bulb. In the case of uninterrupted operation of these lamps, therefore, the above described electron emission material disappears within a few hundred hours after starting operation. The radiant efficiency drops accordingly, and as a result, a low pressure discharge lamp cannot obtained in which high radiant efficiency is obtained over a long time.